This invention relates to a photocurable composition, more particularly to a photocurable composition which has good photocurability and rapid curability as well as excellent coating workability such as flowability, smoothness and uniformity of the coated film, and can also give a cured product having excellent adhesiveness, coating performance, heat resistance and weathering resistance.
Photocurable resin compositions, having generally advantages in aspects of conservation of energy, conservation of resources, saving of space and counter-measures against pollution, are now under aggressive researches and developments in various fields such as ink, paint and varnish. Accordingly, demands in aspect of performance corresponding to respective fields are also increasing.
For example, in photocurable resin compositions to be used for paint, it is desired to improve physical properties of the coated film, particularly adhesiveness, heat resistance and toughness. However, in resin compositions based on acrylic resins, while having an advantage of rapid curing speed, there is involved the drawback of poor adhesiveness. Also, for improvement of coated film performance of said acrylic resins, attempts have been made to solve the problem by formulating heat resistant resins or tough resins to copolymerize sid resins with acrylic resins. In any case, while some improvements of respective physical properties may be recognized, the composition will suffer from solidification or thickening to be limited in improvements of respective physical properties. Thus, the problem remains with respect to working characteristics.
More specifically, one proposal is a photocurable composition comprising an epoxy resin and an acrylic (methacrylic) resin. The specific feature of the photocuring mechanism in this composition resides in modifying the epoxy resin with acrylic acid, etc. in order to introduce acryloyl groups having photocurability into the epoxy resin and photopolymerizing the vinyl groups introduced. However, the cured product obtained from this composition had the drawback of being lowered in characteristics such as heat resistance and adhesiveness as compared with the epoxy resin itself [see "The Present Situation and Future Development of UV-curable Paint" in Coating & Paint, May (No. 324), p. 45, 1980].
Another known composition is a photocurable composition comprising an epoxy resin and a photocurable catalyst of the type releasing strong acid (see Macromolecules, Vol. 10, p. 1037, 1977).
The specific feature of photocuring mechanism in this composition resides in curing the composition by utilizing the photolytic activity of the catalyst represented by the formula: ##STR1## wherein Ar represents a group such as phenyl, X an iodine atom, a sulfur atom or a diazo group and Y BF.sub.4, PF.sub.6, AsP.sub.6 or SbF.sub.6.
However, in the cured product obtained from this composition, while having good mechanical characteristics and heat resistance, there are involved the drawbacks such the characteristics of the cured product will be deteriorated by heating or with lapse of time, and also that corrosive phenomenon of the cured product will be brought about, because the catalyst component is a strong acid which will become an ionic impurity. Also, this composition had a great drawback in difficult handleability, and also a drawback of having no adhesive force at all when cured rapidly.